Regional hyperthermia is a promising new modality of anticancer therapy; but since hyperthermia can also damage normal tissues, further progress in the field requires development of devices capable of focusing heat into tumors or tumor bearing organs without damage to adjacent structures. While considerable progress has been made in the focal heating of tumors located at or near the skin surface, the focal heating of deeply seated tumors has been a more elusive goal. This problem results largely from the fact that electromagnetic radiation in that portion of the frequency spectrum which has deep penetrance in biologic tissues does not have quasi-optic properties while higher frequency EM radiation has quasi-optic properties but limited penetrance.
Therefore, well-focused hyperthermia at depths of more than 5-6 cm below the skin probably requires at least partial use of invasive methods because of constraints resulting from the physical interactions of electromagnetic waves and biologic systems. Safety considerations suggest that invasive devices with sharply focused heating patterns may be preferred for deep hyperthermia. Sharply focused heating also permits temperature monitoring using relatively few invasive temperature probes.
Currently used invasive methods, such as interstitial antennae or RF electrodes designed to accomplish focal heating of surface tumors, have several limitations in regard to heating deeply seated lesions. In the case of the interstitial antenna, the depth of insertion, which depends upon the length of the antenna, is not arbitrarily variable, since for optimum efficiency the antenna length should be some multiple of the incident wavelength. Also, the size of the heat deposition pattern in the vicinity of the antenna tip is significantly limited due to the characteristic interaction of the antenna and the medium, and this is not easily varied. In the case of RF electrodes, the current paths are dictated by the locations of the source and the sink electrodes and quite often require several electrodes to conform to the approximate contour of the desired heating volume (tumor). The use of multiple electrodes increases the risk of bleeding and tumor re-implantation.